A. Field of the Invention
The present invention relates to computer disk drives and more particularly to headerless-formatted data storage on a platter of a disk drive.
B. Background of the Invention
A typical computer disk drive media platter used for data storage is shown in FIG. 1. Platter 100 contains embedded servo fields S1-S4 that extend radially from aperture 102. In a typical disk, there may be 60-80 servo fields, or more. The servos do not have to be aligned. Platter 100 also includes annular zones Z1 and Z2. Zone Z1 is defined by aperture 102 and circumference 104. Zone Z2 is defined by circumferences 104 and 106. In a typical disk there may be many more zones. Typically, each zone contains multiple tracks, and each track is divided into multiple sectors. As shown in FIG. 1, zone Z1 and Z2 each include one track. The track of zone Z1 is divided into sectors 1-6. The track of zone Z2 is divided into sectors 1-11.
Servo fields S1-S4 are used to position the heads of the disk drive for read/write operations. The servo fields can contain data such as the specific servo field, the cylinder, and the track of the platter. Data are stored on platter 100 between servo fields S1-S4 in the various sectors. Occasionally a sector on platter 100 will be divided into one or more sector fragments by a servo field.
The data is recorded in a density defined as bits-per-inch (BPI). The BPI remains generally uniform over the entire area of platter 100. Tracks located radially farther from aperture 110 than other tracks will have a greater length. For example, the track of Zone Z2 has a larger radius as the track of Zone Z1 and therefore the track of Zone Z1 will have a greater length. Consequently, the track of Zone Z2 will be able to store more data than the track of Zone Z1.
As the platter 100 rotates, clock pulses are provided to a disk controller by the read/write channel. The clock pulses generated by the disk controller clock are synchronized to the various fields on the platter 100. A servo pulse is typically generated during or at the end of a servo field. A sector pulse is generated at the beginning of each sector. An index pulse is generated once every platter revolution to coincide with the index field on the platter indicated by I in zones Z1 and Z2.
Each sector is identified by a physical sector address (PSA). Some disk drive systems store identification data within a header portion of each sector. This is referred to as a headered format. Other methods of data storage provide a headerless system, where the sector identification and format characteristics are stored in memory, rather than on the disk. This is referred to as a headerless format. FIG. 2 depicts a headerless sector divided into various fields.
The headerless format reduces the amount of overhead within each sector. That is, ID header fields, header CRC fields and the like, which decrease the amount of platter space available for data storage, are omitted. In its place, the headerless disk controller determines sector locations based on the servo fields, as shown in FIG. 1, in combination with address information stored in memory. The radially extending servo fields S1, S2, S3, S4 are equally spaced around the circumference of the disk 100. Even though the width between the servo fields increases as one proceeds from the inner most servo sector to an outermost servo sector, the time between the occurrence of each servo remains constant. That is, servos occur at regular intervals regardless of the radial distance at which the read head detects them. In addition, the number of servo sectors per track remains constant irrespective of the data track.
Typically, the headerless sector fields include a pad 202, which is a buffer zone that is written to the disk to ensure the magnetization of the disk medium, specifically, its magnitudes and frequency content, is within normal parameters. Following the pad 202 is a PLO field 204. The PLO field 204 includes bytes having a known pattern that facilitate clock recovery. The pattern of bytes is typically locked onto by a phase locked loop circuit in a disk controller for synchronization to the data stream from platter 100. An equalization field 206 typically follows the PLO field 204. SYNC field 208 includes a readily identifiable predetermined pattern to allow the disk controller to align to the beginning of data field 210. A second pad field 210 is included as a buffer at the end of the data field.
The disk controller obtains information regarding the locations of data sectors from a timing generator, often referred to as a servo timing generator. The servo timing generator typically provides pulses that coincide with the location of the beginning of a data sector--these are referred to as sector pulses. In a write operation, the disk controller responds to the occurrence of a sector pulse by initiating the transfer of data. A certain latency period exists before the controller can begin providing data. The latency may be due to the fact that the disk controller, or sequencer, is running off a clock that is not synchronized to the clock used to generate the sector pulses. In addition, the disk controller, sequencer, and/or associated circuitry may require one or more clock cycles to process the occurrence of a sector pulse. The latency period can cause gaps following a sector pulse, resulting in under utilization of the disk.